A known security device for verifying an item's authenticity is a watermark. Watermarks or signatures are typically produced by utilizing semantic information of the item to be protected, for example, alphanumeric characters, physical features, etc. or other related information (e.g. ownership information). These signatures or watermarks are typically kept with, or incorporated into the protected item. For example, a watermark may be printed within the substrate of a negotiable instrument which includes information regarding the value and the originator of the instrument.
Various digital watermarking techniques are known for both still and video images. For example, reference in this regard may be had to Hartung et al., “Digital Watermarking of Raw and Compressed Video”, Systems for Video Communication, October 1996, pp. 205-213 and Hartung et al., “Watermarking of MPEG-2 Encoded Video Without Decoding and Re-encoding”, Proceedings of SPIE 3020, Multimedia Computing and Networking 97 (MMCN 97), February 1997.
Some of the techniques discussed in these papers include separately coding the image and a watermark image using a pseudorandom number generator and a discrete cosine transform (DCT) to form coded blocks, one of the image to be watermarked and the other of the watermark itself. The DCT coefficients representing the coded watermark block and the coded image block are then added together to form a combined block thus digitally watermarking the image.
Reference may also be had to U.S. Pat. No. 6,037,984, entitled “Method and Apparatus for Embedding a Watermark into a Digital Image or Image Sequence,” by Isnardi et al., issued Mar. 14, 2000. This patent discloses watermarking an image or sequence of images using a DCT unit and quanitizer. The patent discloses generating an array of quantized DCT coefficients and watermarking the array by selecting certain ones of the DCT coefficients and replacing them with zero values. The masked array is further processed by a watermark inserter that replaces the zero valued coefficients with predefined watermark coefficients to form a watermarked array of DCT coefficients, that is, a watermarked image.
Reference can also be had to U.S. Pat. No. 5,841,978, entitled “Network Linking Method Using Steganographically Embedded Data Objects” by Rhoads, issued Nov. 24, 1998. This patent describes a data object containing both a graphical representation to a network user and embedded information, such as the URL address of another network node, to enable the object itself to serve as an automated hot link.
An example of a commercially available digital watermarking system can be obtained from Digimarc Corporation (www.digimarc.com), which is shipped with Adobe Photoshop(tm).
By way of summary, digital watermarking involves taking a digital representation of some information and altering it such that a final sensing device, whether electronic or human, will not notice the alteration without using some specific type of decoding device. The decoding device may be implemented as a combination of hardware and software.